This application discloses an invention which is related, generally and in various embodiments, to the mobile storage and transportation of compressed natural gas. Natural gas use is rapidly increasing, and various methods of transporting the gas have been developed. The conventional methods of transporting natural gas in industrial quantities include (1) pipelines which transport the gas in its gaseous form at high, moderate and low pressures, (2) ships, tanker trailers, and ISO containers which transport the gas in its liquid form as liquefied natural gas (LNG), and (3) tube trailers and freight vessels which transport the gas in small (6 to 8 niches) to medium sized (10 to 18 inches) diameter tubes as compressed natural gas (CNG).
Each method of transporting natural gas has associated system design costs and other commercial cost which are contained by factors including number of gas users and user locations. Before selecting the most suitable method of transporting natural gas, an analysis is performed to evaluate the effect of the various factors on the cost of the system and ultimately the price of the gas to users. The analysis takes into consideration factors such as source gas location, distance to user location, gas capacity required, transportation cost, and government regulations.
Large diameter (24 to 36 inches), high pressure pipelines have been the major method of going large gas capacities from source to city-gates for re-distribution to users. From the city-gate, the gas is re-distributed to users via smaller diameter low pressure pipelines. In many cases pipeline gas distribution is not economical, particularly if the gas source and pipelines are located far from user locations.
Processing and transporting the gas as LNG has proven to be a more efficient method of getting large quantities of gas to distant locations where pipeline use is cost prohibitive or impractical. However, LNG production facilities can be quite expensive to construct, and require extensive training and expertise for operations. In addition, the LNG transportation systems can also prove quite expensive and these cost drivers have forced the economics around LNG use to favor large gas capacity users such as power plants and manufacturing facilities.
In some cases, transporting natural gas in its compressed form as CNG can be cost effective. This holds true when gas is needed in locations where there are no pipelines, and no access to LNG.
There are several methods and systems for transporting CNG in use today. Systems include tube trailers, marine Coselle (a trademark of SEA NG) vessels, and small capacity CNG systems serving as virtual pipelines for fuel stations. Of these, the systems used primarily to supply natural gas in industrial capacities are the Coselle systems, and the tube trailer system. The Coselle system which utilizes coiled pipes has a net gas carrying capacity of about 4.2 million standard cubic foot (set) at 4000 pounds per square inch (psi) and is specifically designed for transporting gas via marine vessels. Tube trailers typically range is gas carrying capacities as low as 10,000 scf to as high as 170,000 scf. Individual trailer tubes are typically designed to contain gas pressures of up to 3600 psi and typically vary in diameter from 6 to 18 inches. Tubes are arrayed in a tube bundle and secured onto the trailer, and are generally of similar length as the trailer. The small capacity CNG systems serving as virtual pipelines for fuel stations has gas capacities ranging from about 38,000 scf to 54,000 scf.
These CNG storage and transportation systems have proven useful, but are not without some noted drawbacks. The large capacity Coselle systems are more suited for large industrial operations and are not designed to support smaller gas capacity markets. Also, the Coselle units are designed for marine transportation and are often operated from marine vessels. Smaller capacity tube trailer CNG storage and transportation systems, designed to support smaller gas capacity markets, also have some drawbacks as well. Most notable among these include the cost of the trailer to which the tubes are secured, regardless of capacities. The cost of the trailer increases the cost of the CNG operations because the tube bundle is secured to the trailer, thus requiring a trailer for each tube bundle. Other smaller capacity CNG storage and transportation systems which do not have the drawbacks of the Coselle or the tube trailer have other drawbacks. Most noted among these is their small gas capacity, 38,000 scf to 54,000 scf. This small capacity limits their application, increases their turn-around time, and increases the cost of a CNG operation using them.